1. Field of the Invention
The present invention relates to a novel, efficient, process for the preparation of an alkene interpolymer comprising polymerizing at least one 3-substituted C4-10 alkene and another C2-8 alkene in a gas phase polymerization using a polymerization catalyst system. The invention also relates to interpolymers obtainable from the process.
2. Description of the Related Art
Alkenes, such as ethylene, are often copolymerized with comonomers in order to obtain polymers having particular properties. Thus it is common to copolymerize ethylene with comonomers such as 1-hexene or 1-octene in order to obtain a polymer having, for example, decreased density relative to ethylene homopolymer. Decreasing the density of the interpolymer generally impacts positively on a number of its mechanical properties, potentially making the polymer more useful in a number of end applications. Thus comonomers are generally used to tailor the properties of a polymer to suit its target application. There are vast numbers of commercially available ethylene interpolymers, e.g. comprising 1-butene, 1-hexene or 1-octene as comonomers.
A significant proportion of alkene polymer, e.g. polyethylene, is produced industrially using gas phase polymerization. Gas phase polymerization has several advantages over slurry polymerization. First there is no need for a slurry diluent, which in slurry polymerization is a component that is present in a large amount in the production plant, but which must be separated from the polymer at the end of the polymerization process and recovered and purified for reuse. Second the drying of the polymer after a gas phase polymerization compared to a slurry phase polymerization is much simpler. Third polymer powders that are too sticky to handle in slurry polymerization may behave well in gas phase polymerization, e.g. polymers having a density of 920 kg/m3 may be too sticky and soluble to be produced in slurry polymerization, while corresponding polymers having a density of 910 kg/m3 may be easily produced in gas phase polymerization. Fourth, in the case of a multistage process wherein much less comonomer were to be required in the second step, in the case of a gas phase reactor, there would be no need to remove comonomer from the polymer flow between reactors.
Gas phase polymerization may be carried out using any conventional polymerization catalyst system, e.g. a Ziegler Natta, single site or chromium oxide (Phillips-type) containing catalyst system. The catalyst system chosen is largely dictated by what properties are desired in the final interpolymer. For example, if an interpolymer with good processing properties is desired, the skilled man is likely to choose a Ziegler Natta catalyst or a chromium oxide catalyst. On the other hand, if the key desired property of the interpolymer is that it be homogenous, the skilled man would probably choose a catalyst system comprising a single site catalyst.
Regardless of the nature of the polymerization catalyst system used, when gas phase polymerization is carried out industrially it is usually conducted as a continuous process because this is economically most attractive. Thus the polymerization catalyst system is continuously introduced into the gas phase reactor along with the appropriate monomers, whilst the desired polymer is continuously removed. The continuous addition of fresh catalyst system is necessary because when the desired polyalkene is removed from the reactor system, a certain amount of catalyst system is also removed. It is thus important to provide additional catalyst system in order to maintain the polymerization reaction.
A disadvantage of this manufacturing set up, however, is that the catalyst system that is removed from the reactor with the desired polymer cannot usually be separated therefrom. Rather the catalyst system will typically be present within the polymer in the form of a partially modified residue. In other words, the catalyst system is present in the polyalkene as an impurity.
The presence of catalyst system residues in polymers such as polyethylene is undesirable for a number of reasons, e.g.                they make processing, e.g. to fibers or films difficult if the residues make particles of the same size or greater than the fiber or film thickness        they reduce the performance of the polymer in its end use, e.g. it can reduce the optical performance of films made using the polymer by making visually observable inhomogeneities in the film, often called gels, specs or fish eyes        they can render polymers unsuitable for use in applications where the level of impurities present therein is required to be below a certain standard, e.g. in food and/or medical applications        they, through their content of transition metals, can act as accelerators for polymer degradation resulting eventually in discoloration and loss of mechanical strength.        
It is thus generally desirable to try to minimize the amount of catalyst system needed to make a given amount of polymer. This helps overcome the above-mentioned problems in processing and use and also decreases the production cost of the polymer through reduced catalyst system cost per ton polymer. It also minimizes any safety risks associated with the handling of catalytic materials. Additionally the ability to use a lesser amount of catalyst system per kg of final polymer in some cases enables production plants to increase their production rate without having to increase their reactor size.
There are a number of known methods that usually would increase catalyst system productivity (i.e. ton polymer/kg catalyst system) for a given catalyst system. These include increasing the residence time in the reactor, increasing the polymerization temperature, increasing the partial pressure of monomer and/or the partial pressure of comonomer. All of these approaches, however, suffer from serious drawbacks.
Increasing the residence time can only be done by decreasing production rate, which is economically unfavorable, or by increasing polymer concentration in the reactor which may easily lead to fouling and/or lumps in the reactor and ultimately to a long stop for cleaning. Increasing the partial pressure of monomer has a negative effect on production economy by reducing the relative conversion of monomer. Increasing the partial pressure of comonomer increases the incorporation of comonomer and thus, in effect, leads to the production of a different interpolymer to the one targeted. Increasing the polymerization temperature from the usual operation temperature is probably the most common strategy employed to date, but as with increasing the residence time it can lead to reactor sheeting or lumping or chunking in the reactor and again to a long stop for cleaning the reactor system.